Polymer printing

ABSTRACT

Improved printing with polymers is achieved by interposing a liquid layer conforming to rough substrate surfaces and possibly exhibiting different characteristics from an outer surface polymer layer. This is particularly advantageous in printed wiring board (PWB) manufacture requiring solder mask coating. Thus, a two layer composite polymer coating is provided. One inner adhesive photopolymer layer is applied to the PWB in the liquid state, displacing air from PWB surface. The outer layer of the composite therefore can be epoxy, dry film or liquid polymer. Dry film thus carried on a thin plastic sheet may be overlaminated onto a liquid inner layer already on the substrate, without the need for a vacuum laminator to eliminate bubbles or a plasticizing heat step to conform the dry film to the surface. The dry film solder mask so laminated is then exposed through a phototransparency to harden the light struck dry film solder mask and light struck inner layer photopolymer, thereby cojoining the dry film solder mask, inner layer and PWB surface. A solvent washout step removes unexposed dry film solder mask and unexposed inner layer photopolymer. This provides a faster process, requires less equipment, and improves adhesion to metal conductors. This embodiment gives a high resolution product with good solder mask qualities. The outer layer need not be photoimaged. Thus, pre-patterned epoxy solder masks may be printed on the carrier sheet, partially hardened and overlaminated in registor onto liquid photopolymer-coated PWB, then exposed to light source through a phototransparency and through the pre-patterned solder mask, thereby permanently adhering the outer layer to PWB. The composite coating can be a combination of known solder mask materials, dry film, UV-curable liquid polymers and thermal-curing epoxy, chosen for desired characteristics including electrical performance, printing resolution and cost.

This is a continuation-in-part of my co-pending application Ser. No.364,309 for Printed Wiring Board, filed Apr. 1, 1982, now U.S. Pat. No.4,506,004 which was reissued on June 2, 1987 as U.S. Pat. No. Re.32,430.

TECHNICAL FIELD

This invention relates to printing with liquid polymers and moreparticularly to registration of patterns on a substrate surface, such asa printed wiring board, by means of an intermediate liquid polymerlayer.

BACKGROUND ART

Typically, polymers are used in the manufacture of printed wiring boards(PWB), by coating the PWB with a polymer coating. For solder masks thecoating pattern covers conductor wiring traces at all locations exceptwhere electrical contact is to be made with the traces. The curingtraces may also be printed on a metallic layer such as copper on aninsulating substrate by means of polymer printing patterns definingareas which are to be etched in one primary imaging method. In otherprimary imaging methods, polymer printing patterns may define zones foraccumulation of conductive materials forming conductor traces. Otherprinted objects are similarly manufactured by use of the polymerprinting patterns.

In common use are three types of polymers, namely, (1) thermal curingepoxy, (2) ultra-violet (UV)-curing polymers generally classified asliquid polymers which are hardened by radiation, and (3) dry filmpolymers which are changed in solubility by radiation. The epoxies andUV-curables are applied as liquid coatings for curing in place ascontrasted with the dry films.

Some of the patterns formed by these polymers are formed mechanicallysuch as by silk screening. Others are formed by photo-imagingtechniques, where the photo response characteristics of certain polymerssuch as the UV curing liquid polymers are utilized.

Dry film solder mask photopolymers are supplied by two manufacturers, inroll form, consisting of an inner layer of photopolymer sandwichedbetween a carrier film of clear polyester and a polyolefin liner. Theyhave problems of adhering to copper, and are not conveniently used foroverlaying rough surfaces. Thus, for a solder mask coating on a typicalPWB, wherein the metal conductors extend 0.003 to 0.004 inch above thebase laminate, the dry film needs be conformed without intermediate airbubbles. The use of a roller laminator is not usually satisfactory, asair is trapped between the photopolymer and the PWB laminate,particularly between closely-spaced conductors. A vacuum chamberlaminator is normally used in a plasticizing procedure to prevent airentrapment. The lamination cycle is as follows: A PWB and a section ofdry film solder mask is inserted into the heated chamber; the chamber isevacuated, and when up to temperature the photopolymer is forced intocontact with the PWB, effecting an air-free lamination. Laminationoccurs at a temperature of the order of 200 degrees F., at whichtemperature the dry film solder mask photopolymer becomes tacky andadheres to the PWB surface. The solder mask pattern is attained byexposing the photopolymer to a strong UV light source through aphotographic film, wherein the light hardens the exposed photopolymer.The polyester carrier film is then peeled away and the unhardenedphotopolymer is washed out in a solvent spray bath.

There are several shortcomings with the dry films listed below:

1. The single photopolymer layer is expensive being of the order of fourtimes the cost of liquid photopolymers; the equipment required tolaminate is overly complicated and expensive; the labor required isexcessive, as each processing step of laminating, exposing, anddeveloping is overly lengthy.

2. The laminating step forces photopolymer into circuit holes, and thelike, with the result that with small holes of the order of 0.025 inchdiameter, the photopolymer does not wash out, leaving the holes plugged.

3. The laminating step forces photopolymer into large tooling holes andslots, leaving a puckered, striated coating of photopolymer, for thepolyester carrier film is non-conforming and wrinkles around largerholes and sharp corners.

4. The dry photopolymer is characterized by a lack of adhesion to metalconductors. For PWB having bare copper conductors with dry film soldermask thereover, the solder coating and hot-air leveling step is usuallynot satisfactory without copper surface pretreatment as with a blackoxide coating; otherwise the solder mask separates from the conductors.

5. The dry photopolymers need be applied in thick layers of standardthicknesses, and the thicker layers are not only expensive but alsoinhibit high resolution photo reproduction and require highertemperatures to photodevelop.

6. The photopolymer is temperature sensitive, delaying the applicationsequence. After laminating at 200 degrees F., the photopolymer must coolto room temperature prior to exposure. After exposure, which raises thetemperature again, the photopolymer must cool to room temperature beforewashing out unexposed photopolymer.

Similarly epoxy coatings have disadvantages since the initial liquidlayers required need be cured in situ, generally at very hightemperatures which can damage the printed wiring boards or othersubstrates being printed upon.

The liquid polymers provide problems in initial application since theydo not tend to stay in place, and have conventionally been applied invery thin coatings, unsuitable for pinhole free coatings, and forcovering the rough surfaces of printed wiring traces.

The different polymers each have advantageous features, but it is notknown in the prior art how to overcome this disadvantage in producingprinted patterns on substrates therewith.

Typical of prior art polymer printing techniques used in PWB are thoseof U.S. Pat. No. 3,629,036--C. Isaacson, Dec. 21, 1971, where amicrothin adhesive layer including a photopolymer solvent isinterspersed between smooth copper surfaces and a dry film photoresistlayer, which is thereby glued in place before photoimaging anddeveloping the desired pattern.

Reference is made to U.S. Pat. No. 3,824,104 in which Kloczewski teachesa method for photoimaging a liquid photopolymer, wherein the imagebearing photomask is separated from the photopolymer by a distance ofeight mils during exposure, leading to a stated loss of resolution.

Reference is made to U.S. Pat. No. 4,260,675 in which Sullivan describesa method for photoimaging liquid photopolymer using a glass platephotomask with raised opaque pillars in contact only with portions ofthe PWB which will be free of hardened solder mask.

Each of these patents and current practices in the art of PWBmanufacture has characteristics which this invention seeks to improve.

One objective is to improve the adhesion of dry film to metalconductors.

A second objective is a process to laminate a dry film to a highlyirregular surface such as over PWB traces without use of a vacuumlaminator.

Another objective is to reduce the time required to process dry film bynegating the requirement for laminating at an elevated temperature.

Another objective is to achieve a photoimaged solder mask having theelectrical and environmental characteristics of dry film, but at reducedcost.

Another objective is to achieve a photoimaged PWB solder mask in whichUV-curable liquids are exposed with a photomask in contact with theliquid, and in which the solder mask coating is not thinned out over themetal conductors.

DISCLOSURE OF THE INVENTION

This disclosure teaches an improved method for polymer printing using acontact layer of a liquid polymer and an outer layer of one of (1) dryfilm polymer, (2) UV-curable liquid polymer, or (3) dry epoxy polymer.

The combinations of two polymer layers yield a wide range of compositecoating characteristics, including electrical performance, environmentalreaction, printing placement accuracy, adhesion and cost.

Before the dry film solder mask is applied to the PWB, a coating ofliquid photopolymer is screen printed over the entire PWB surface so asto coat both the circuit traces and the base laminate, and in so doingall air is displaced from the PWB surface by the liquid polymer. The dryfilm solder mask photopolymer is then laminated onto the liquid polymercoated PWB without the need for a vacuum laminator.

In one embodiment, the composite polymer coating is then exposed to astrong UV light source through a photographic film mask, with lightstruck areas being hardened sufficiently to withstand the subsequentsolvent washout step.

When the outer layer is formed from a UV-curable liquid, then theprocess includes the formation of a semi-cured coating on animage-bearing transparency and the over-lamination of the transparencyonto the coated PWB in register. A second exposure step bonds the twolayers together and to the PWB. The formation of the semi-cured coatingprevents photopolymer thin-out or starvation on the raised metalconductors.

When the outer layer is a dry epoxy polymer that is formed, dried andcarried on a thin plastic sheet, then the process is the same as for dryfilm.

When the outer layer is a non-imaging dry epoxy coating carried on athin transparent sheet, then the procedure is to apply the coating tothe transparent sheet in the liquid state by screen printing the desiredimage, then curing to a semi-cured state, and then laminating to thecoated PWB. When the inner layer is exposed to a light source throughthe outer coating and through an image bearing transparency, then theinner layer thus exposed bonds the epoxy outer layer thereto. Unexposedinner layer photopolymer is washed out in a solvent and the compositemask completed by a final curing step.

Essential to all these embodiments is the liquid polymer active layerdisposed in intermediate surface contact between an outer polymer layerand a substrate surface to be printed, such as a printed wiring board.This liquid layer is disposed in paste like consistency to avoidbubbling and entrapping bubbles and for remaining in situ withoutrunning off or into holes, etc. Thus, the objects coated may be handled,and placed vertically, etc. without changing the thickness or placementof the liquid polymer, whether it be patterned, such as from a patternedsilk screen deposit on the laminate, or simply forms a constantthickness layer from an unpatterned silk screen deposit, or otherdesired method.

When the two layers are of different polymers, the inner and outerlayers may be provided with differing properties to achieve differentobjectives with the compound polymer layer, such as better adherance,better heat characteristics, lower cost, better resolution, thinner orthicker coatings, etc. Similarly the layers may be processable withdifferent solvents thereby providing for selective removal of specifiedlayers only, producing a rich range of novel techniques for achievingprinted products.

THE DRAWING

In the drawing:

FIG. 1 is a sectional cut-away showing lamination apparatus for coatinga printed wiring board with two polymer layers, the inner of which isliquid polymer;

FIG. 2 is a diagram showing the procedure for producing the dry filmsolder mask - UV-curable solder mask composites of one embodiment of theinvention; and

FIG. 3 is a diagram showing the procedure for a further embodiment forproducing solder masks with UV-curable photopolymers.

PREFERRED EMBODIMENTS

A brief description of the physical and electrical characteristics ofvarious polymer materials is discussed so that the advantage afforded bythe disclosed process will be better understood. Table 1 compares thegeneralized properties of three major types of solder mask materials:screen-printed epoxies, screen-printed UV-curable photopolymers, and dryfilm solder masks, with the disclosed composite dry film - UV-curablephotopolymer solder mask afforded by this invention.

                                      TABLE 1                                     __________________________________________________________________________                           Screen Printed Dry Film Solder Mask                                   Screen Printed                                                                        UV-Curable                                                                            Dry Film                                                                             Composite With Liquid                   Property       Epoxy   Solder Mask                                                                           Solder Mask                                                                          Inner Layer                             __________________________________________________________________________      Adhesion     Excellent                                                                             Fair    Poor   Excellent                                 Environmental                                                                              Excellent                                                                             Fair    Excellent                                                                            Excellent                                 Electrical   Excellent                                                                             Fair    Excellent                                                                            Excellent                                 Printing Resolution                                                                        Poor    Poor    Excellent                                                                            Excellent                                 Material Cost Per Unit                                                                     1         1.5   6      4                                         Area                                                                          Application Time                                                                             1.25  1       5      2                                         Application Equipment                                                                      1       4       20     5                                         Cost                                                                          Total Application Cost                                                                     1       1       5      2                                         Application Operator Skill                                                                 Highly  Highly  Unskilled                                                                            Unskilled                                 Level Required                                                                             Skilled Skilled                                                __________________________________________________________________________

As shown in Table 1, the traditional solder mask epoxies are the bestsolder mask materials for PWB when the relatively poor printingresolution can be tolerated, whereas the dry film solder mask isoutstanding in electrical and environmental characteristics and photoresolution, and requires little operator skill. Dry films, however, arecostly and suffer poor adhesion. However, in accordance with thisinvention, a coating of liquid polymer in paste like consistency isapplied over the entire substrate surface prior to dry film application.This results in an air-free lamination effected with relativelyinexpensive equipment such as by a scanning squeegee, while the adhesionof the composite is improved over dry film alone. The electrical andenvironmental characteristics are both excellent, as determinedprimarily by the dry film outer coating. Material cost is reduced, forinstead of a required 3 mil dry film thickness, the composite can be 1mil of the expensive dry film and 1 mil of inexpensive liquid polymer. Asignificant further advantage is that photoresolution for photoimagingphotopolymers is better for thinner layers, which produce lessundercutting during washout.

The main thrust of this invention is a composite coating having a liquidpolymer inner layer. A particularly advantageous embodiment provides atwo-layer composite in which air sensitive thermal-curing polymers areformed on a thin plastic sheet with one side exposed to air where theybecome partially-cured through their thickness presenting a liquid layerfacing the air side. This liquid side then can cover rough surfaces on atransfer PWB, for example. The outer hardened layer then assures thatthe outer layer thickness is superimposed over wiring traces to preventstarving.

As used in this disclosure a liquid polymer has a range of viscositiesfrom that of a fluid to heavy paste-consistency; viscosities normallyencountered in liquids used in screen printing, roller coating, andcurtain coating.

EXAMPLE 1 Dry Film Photopolymer Covering Liquid Photopolymer Inner Layer

Reference is made to FIG. 2, showing the steps to be followed to achievea composite photoimaged solder mask coating. Step 1 is to coat the PWBhaving traces 0.004 inch high with a liquid polymer coating 0.001 inchthick. A polyester or nylon screen printing fabric of 125 mesh producesthe 0.001 inch coating. The liquid polymer is the W. R. Grace "Unimask1000" UV-curable solder mask photopolymer which is 100 percent solidsand does not outgas during curing.

The dry film solder mask is prepared for laminating to the coated PWB. Asection of Dupont "720S Vacrel" dry film with polyolefin liner film istemporarily adhered to the underside of a second screen printing framehaving a nylon or polyester fabric of 235 mesh with the polyestercarrier film in contact with the screen. The adhesive is the 3M "SprayMount" adhesive. After adhering, the polyolefin liner film is removed.

Lamination, Step 4, is accomplished by placing the coated PWB under theprinting frame with affixed dry film off-contact, then drawing asqueegee blade the length of the PWB across the top side of the fabric.This step forces the dry film into intimate contact with the polymer insuch a way that no air is trapped.

Reference is made to FIG. 1, a partial sectional cutaway drawing of thePWB being laminated. PWB 11 with metal conductors 13 rests on surface12. Screen frame members 10 tension and restrain polyester fabric 18.Dry film photopolymer 14 is shown mated with the PWB behind blade 15 andadhered to screen fabric 18 ahead of the blade. Force 16 causes blade 15to move downward, forcing photopolymer 14 to contact liquid polymerlayer 19. Force 17 causes blade 15 to scan the length of PWB 11.Photopolymer 14 is temporarily adhered to fabric 18 by a light adhesivecoating, not shown in FIG. 1.

After lamination, the composite is moved away from fabric 18 and thenexposed to a strong UV light source through a photographic filmpositive, thereby hardening both the outer dry film solder mask and theunderlying liquid photopolymer. For example, the Colight "DMVL-HP"exposes the composite in five minutes. After exposure, the uppermostpolyester carrier sheet (not shown) is peeled away from the dry filmphotopolymer. Unhardened photopolymer is washed out in a Dupont "A24"processor using trichlorethane spray for a period of two minutes. Liquidpolymer 19 not covered by hardened photopolymer is also washed away bythe trichloroethane.

There may be some undercutting of the inner layer photopolymer by thetrichloroethane, leaving a slight annular ring around openings in thedry film solder mask where the underlying photopolymer is washed away.This condition is largely eliminated by using the Dupont aqueous dryfilm solder mask, in which the unexposed photopolymer is washed out in aDupont proprietary water-based solvent. This solvent does not remove theinner layer. A second washout bath of trichloroethane for 20 secondsserves to washout unwanted inner layer photopolymer cleanly withoutsignificant undercutting of the hardened dry film solder mask. Thisillustrates a significant advantage of the invention which permits twodifferent polymers or two different polymer states, cured and uncured,to be processed independently by selective use of solvents for attainingdifferent results on corresponding layers.

The photoexposure step permits development of the desired patterns by aninitial cure changing the state of the polymer. However, particularlyfor solder mask coatings that need withstand high temperatures, a finalhard cure of the composite film is desirable. This is accomplished byexposure to a greater cured radiation such as provided by a strong UVlamp source to fully cure the photopolymer. A conveyorized UV curingunit having two lamps, each rated at 200 watts per linear inch cures thephotopolymer at a speed of five feet per minute. Argus International ofHopewell, N. J. manufactures a suitable UV curing unit.

While the aforementioned UV-curable solder mask photopolymer, the"UNIMASK 1000", can be used as the inner layer, the five-minute exposurecycle can be reduced by use of a photopolymer having a higherphotospeed. Such a photopolymer can be formulated by combining thefollowing:

CELANESE RADIATION CURABLE RESIN "RR-27418": 100 PARTS

BENZOPHENONE: 3 PARTS

N-METHYLDIETHANOLAMINE: 2 PARTS

EXAMPLE 2 UV-Curable Solder Mask Covering Liquid Photopolymer InnerLayer

When photoimaging a liquid photopolymer, to obtain high resolution, itis preferable to use a phototool contact system, whereby thephotopolymer film is pressed into intimate contact with the photopolymerduring the exposure cycle. When the paste consistency liquidphotopolymer is covering a PWB having a highly irregular surfacetopology, there is the problem of photopolymer starvation of thin-outover the conductors, for pressure on the phototool forces thephotopolymer to extrude sidewards and off the conductors.

Photopolymer starvation in this disclosure is prevented by use of acomposite solder mask, wherein the outer layer is formed on the surfaceof an image-bearing photomask or photographic film positive containingopaque dots on a transparent field for defining solder points. Thisouter layer is deposited onto the photomask surface to a thickness of0.001 inch by screen printing. Exposure to a UV light source through thephotomask partially hardens the light-struck photopolymer to form thedesired pattern. The photomask is then mated with a PWB which has beencoated overall with a thin layer of paste consistency solder maskphotopolymer, in register, so that the phototool opaque areas arepositioned over the PWB circuit conductor pads which are to be free ofhardened solder mask. Mating is preferably accomplished by use of aresilient blade to eliminate the need for vacuum chambers. The partiallyhardened photopolymer does not extrude from the top of the conductorsand thus forms a controlled thickness solder mask outer layer. Theassembly is exposed to a strong UV light source through the matedphotomask, so as to harden both layers of the photopolymer down to thePWB base laminate. The solder mask is completed by removing thephototool and subjecting the PWB to a solvent spray bath to removeunhardened photopolymer and then final curing with a UV curing unit.

It is to be understood that many other advantageous applications aremade available by the partially cured through its thickness polymer,thus provided. This technique itself produces a hardened and similar todry film layer and a liquid layer, so that better adherence over roughsurfaces is ensured. Thus a screen formed pattern, partially cured inthis way can be adhered to various surfaces directly and then finallycured in-situ to assure a permanent bond and fully cured pattern.

FIG. 3 shows the processing steps to be followed for producing a PWBsolder mask coating.

Step 1 is to prepare a special photomask which will have a non-stickrelease surface coating, and which will withstand the high temperatureof the light source, while maintaining dimensional stability. Thephotomask consists of a clear polyester sheet stretched in a four-sidedframe. Photomasks for PWB having higher conductors may be contoured foradditional protection against photopolymer starvation over raisedsurfaces. Contouring is achieved by pressing the phototool onto anuncoated PWB with sufficient force to permanently distort or coin thephotomask lower surface to match the contour of PWB surface.Alternatively, contouring can be achieved by molding the thin releasecoating to the PWB conductor pattern. The opaque areas were previouslyscreen printed onto the polyester sheet.

A thin release coating of Dow Corning "734 RTV" is applied to preventthe photopolymer from sticking. Next, the PWB is coated by screenprinting with W. R. Grace "Unimask 1000" photopolymer, using a 125 meshfabric to deposit a 0.001 inch thickness. The same screen mesh is usedto coat the photomask overall with a 0.001 inch thick coating.

Step 3 is to harden the wanted photopolymer on the photomask. Atwo-minute exposure with the Colight "DMVL-HP" is used.

Steps 4-9 cover the placement of the photomask above and off contact thePWB: mating, reexposure to UV light, solvent washout of unwantedphotopolymer, and final cure. These steps are as previously describedfor the dry film solder mask/liquid photopolymer composite and need notbe repeated here.

EXAMPLE 3 Dry Epoxy Solder Mask Photopolymer Outer Layer

A solder mask epoxy photopolymer known as "Probimer 52" is availablefrom CIBA - GEIGY Company of Ardsley, N.Y. This photopolymer is normallyapplied to the PWB by curtain coating, then oven baked to drive offvolatiles until dry, then photoimaged through a photo transparency tofurther harden wanted photopolymer. Unexposed photopolymer is washed outin a solvent spray bath. This photopolymer is used as the outer layer inthe disclosed process in conjunction with the liquid photopolymer innerlayer.

The disclosed process includes the coating of a thin transparent plasticsheet having a release surface coating with the "Probimer" liquid andheating until the volatiles are driven off. The resulting dry epoxyphotopolymer is processed as described in Example 1 for dry film soldermask. Dupont product "Vacrel 7205". The solvent used to washout theunexposed "Probimer" photopolymer is cyclohexanone.

EXAMPLE 4 Dry Epoxy Solder Mask, Non-Photoimaging

As shown in Table 1, thermal curing epoxies have excellent electricaland environmental characteristics, are low in cost, but exhibitrelatively poor printing resolution. The disclosed two-layer solder maskcomposite using an epoxy outer layer bonded to the PWB by an inner layerof liquid photopolymer retains the primary advantages of the epoxy whileimproving the printing resolution. First, a thin transparent polyestersheet having a release coating is screen printed with catalyzed "PC-401"epoxy in the correct pattern, which is then cured to the point of beingdry but still flexible. To overlaminate the epoxy coating onto thecoated PWB, the thin transparent sheet carrying the epoxy is temporarilyadhered to the underside of a printing screen and laminated onto thecoated PWB, the same way as described in Example 1 for the dry filmsolder mask, the exception being that the epoxy patterns are properlyregistered with the PWB circuitry. At this point in the process, theepoxy outer coating covers the PWB overall except for openings to befree of solder mask such as circuit pads, contact fingers, and toolingholes. Circuit pads are covered by the photopolymer liquid.

To bond the epoxy coating to the PWB, the liquid photopolymer innerlayer is exposed to a light source through the epoxy coating and throughan image-bearing phototransparency. Opaque areas on thephototransparency prevents the photopolymer from being hardened in thosePWB areas to be free of all solder mask.

The solder mask is completed by washing out the unexposed photopolymerin trichloroethane and final curing by exposure to a high intensitymercury vapor lamp to cure the photopolymer, and a bake cycle tocompletely cure the epoxy.

This process improves the solder mask resolution in two ways. First, theepoxy image is printed on a flat surface, wherein the resolution is muchimproved over printing over the irregular PWB surface. Second, the innerlayer is photoimaged, thereby achieving near-exact conformance of soldermask with phototransparency opaque patterns.

Where legends or nomenclaturing is required, the lettering ink isdeposited on the carrier sheet first, and the epoxy solder mask patternsoverprinted then partially cured, firmly adhering the lettering to theouter solder mask layer. Transfer to the coated PWB is effected aspreviously described for patterned dry epoxy.

EXAMPLE 5 Non-Photoimaging Epoxy Inner Layer

The superior adhesion of a thermal-curing epoxy is utilized in thisprocess to bond an outer layer of solder mask polymer to PWB. As inExample 4, the desired solder mask pattern is screen printed onto a thinplastic sheet having a non-stick surface coating to a thickness of 0.001inch. Solder mask epoxy "PC 401" as sold by Nationwide Circuit Productscan be used. This epoxy pattern is partially cured by a bake cycle of250 degrees F for 15 minutes. Next the PWB is coated with a 0.001 inchlayer of Epoxy Technology Company "EPO-TEK 730" catalyzed epoxy, whichis 100 percent solids and does not outgas while curing.

Preparatory to laminating the two epoxy layers, the plastic sheetcarrying the partially cured epoxy pattern is temporarily adhered to theunderside of a printing screen, and placed above and off contact thecoated PWB, in register. A resilient blade drawn across the top surfaceof the printing screen laminates the two together without airentrapment. The plastic carrier sheet is peeled away, leaving the twolayers joined by epoxy adhesion.

The portions of the inner layer not covered by the outer layer is washedout in a trichloroethane spray, while the wanted portions of the innerlayer are protected by the outer layer and are thus shielded from thewashout solvent.

Following washout, a bake cycle of 250 degrees F. bonds the two layerswith the PWB.

In addition to its use in solder masking as described in the fivepreceding examples, the disclosed composite coating has been used by theapplicant to achieve photoimaged plating resist and etch resist patternsused to define the metal conductor patterns on the PWB base laminate.

The process for achieving a composite plating resist or etch resistcoating is as described in Example 2, but substituting the LONDONCHEMICAL COMPANY etch resist 1095 LUV for the UNIMASK 1000 solder mask.In addition to the 1095 LUV, which can be removed from the PWB afteretching, the process described in Example 2 can be used to formpermanent photopolymer coatings using the PPR 101, a permanent platingresist manufactured by the Haven Corporation of Baltimore, Md.

EXAMPLE 6 Differential Patterns in Laminated Layers

The different polymer layers could include transparent and dyed layers,so that two different patterns may be superimposed. Furthermore, twodifferent dyes in the separate layers may give color effects. The dyescould typically filter out curing radiation in the respective layers.Thus, a layer on the substrate could be first photo exposed through onephototool pattern, and the outer layer later superimposed and photoexposed through a separate phototool pattern. The outer layer then couldbe removed by a solvent that does not remove the inner layer, etc.

It is to be understood from the foregoing that this invention providesfor the contact on a printing surface substrate, such as a PWB, of anactive liquid state polymer layer sandwiched between an outer polymerlayer and the substrate, which will form a desired printed image patternin-situ and which will have good adherance qualities over roughsubstrate surfaces, such as printed wiring patterns. Various preferredembodiments and examples are set forth, advancing the state of the art.

Accordingly, those novel features believed descriptive of the nature andthe spirit of the invention are defined with particularity in thefollowing claims.

I claim:
 1. The intermediate product in the method of printing on aprinted wiring board substrate with a metallic surface, a pattern formedby a thin two layered polymer coating, comprising in combination, aliquid polymer layer of paste-like consistency at least about 0.001 inchthickness for forming a significant part of said pattern sandwichedbetween an outer polymer layer partially cured with a predeterminedpattern for forming a significant part of said pattern and said metallicsubstrate surface.
 2. The combination of claim 1 wherein the outer layeris liquid polymer.
 3. The combination of claim 1 wherein the outer layeris a dry epoxy polymer.
 4. The combination of claim 1 wherein the innerlayer is a photopolymer.
 5. The combination of claim 1 wherein the outerlayer is a photopolymer.
 6. The combination of claim 1 wherein thelayers each have a thickness of at least about 0.001 inch.
 7. Thecombination of claim 1 wherein the combined thickness of the two layersis greater than about 0.003 inch.
 8. In the method of producing anintermediate product to be used in the method of printing on a printedwiring board substrate having at least in part a metallic clad surface acomposite pattern formed by two patterns on respective adjacent polymercoating layers, comprising sandwiching an inner layer of liquid polymerwhich hardens by radiation being of significant thickness forming one ofof said two patterns to reside between an outer polymer layer screenprinted in a predetermined pattern shape on a thin plastic sheet forforming the other one of the two patterns and a substrate to which thetwo patterns are to be bonded, with the liquid layer forming a good bondfor producing a desired printed pattern in situ between the outerpolymer layer and the substrate surface without entrapping air.
 9. Themethod defined in claim 8 wherein the outer polymer layer comprises adry epoxy coating formed and dried and carried on said thin plasticsheet, the substrate is coated with a layer of said liquid polymer, andsaid outer layer is laminated upon said liquid polymer layer.
 10. Themethod defined in claim 8 wherein the substrate printed wiring board haswiring traces thereon having a depth of approximately 0.004 inch, andthe two said layers have a thickness of approximately 0.001 inch each.11. In the method of producing an intermediate product to be used in themethod of printing on a substrate a pattern formed by a thin compositetwo layer polymer coating by formation of the thin coating, comprisingthe steps of, sandwiching a layer of liquid polymer forming asignificant portion of said pattern between an outer polymer layer and asubstrate, and forming a good bond between the outer polymer layer andthe substrate surface without compromising desired characteristics ofthe outer layer or entrapping air wherein at least one of the layers hasa patterned shape of partially cured polymer before sandwiching thatprovides the printed pattern shape of that layer.